An echo cancellation apparatus to suppress “acoustic echo” is well known. For example, a speech of a far-end terminal (other terminal) is output from a loud speaker of a near-end terminal (local terminal). Sound waves emitted from the loud speaker are reflected from surrounding objects. A part of them arrive at a microphone of the near-end terminal. It is an acoustic echo. The acoustic echo is captured by the microphone and sent back to the far-end terminal. A far-end speaker hears the echo of his/her own speech with some delay. It causes that conversation is obstructed.
In this echo cancellation apparatus, by using an adaptive filter of which the number of coefficients is N, an echo replica signal y(t) (pseudo acoustic echo) is generated from a reference signal x(t) (a first acoustic signal), which is the received signal of the far-end speech.
From an input signal m(t) (a second acoustic signal) input to the microphone of the near-end terminal and the echo replica signal y(t) (a third acoustic signal), an error signal e(t) (a fourth acoustic signal) is generated using equations (1) and (2). The error signal e(t) is output from a loud speaker of the far-end terminal.
                              e          ⁡                      (            t            )                          =                              m            ⁡                          (              t              )                                -                      y            ⁡                          (              t              )                                                          (        1        )                                                                                    y                ⁡                                  (                  t                  )                                            =                                                ∑                                      k                    =                    0                                                        N                    -                    1                                                  ⁢                                                                  ⁢                                  (                                                            w                      ⁡                                              (                                                  k                          ,                          t                                                )                                                              ·                                          x                      ⁡                                              (                                                  t                          -                          k                                                )                                                                              )                                                                                                        =                                                                    W                    ⁡                                          (                      t                      )                                                        T                                ⁢                                  X                  ⁡                                      (                    t                    )                                                                                                          (        2        )            
The equation (2) represents convolution. “w(k,t)” is k-th coefficient at time t in the adaptive filter. “x(t−k)” is the first acoustic signal at time (t−k).
“W(t)” is a column vector of the coefficients represented by an equation (3). “X(t)” is a column vector of a reference signal represented by an equation (4).W(t)=[w(0,t), w(1,t), . . . , w(N−1, t)]T  (3)X(t)=[x(t), x(t−1), . . . , w(t−N+1)]T  (4)
In the echo cancellation apparatus, by using an adaptive algorithm (For example, LMS algorithm represented by an equation (5)), coefficients w (k,t) are corrected (updated in real time).W(t+1)=W(t)+μ·e(t)·X(t)  (5)
In the equation (5), “μ” is a parameter to determine a degree to correct coefficients of the adaptive filter, which is called a step size. If the step size μ is large, when the second acoustic signal m(t) includes only acoustic echo of the first acoustic signal x(t), the coefficients w(k,t) can be quickly converged, and the acoustic echo can be quickly suppressed.
However, when the second acoustic signal m(t) includes other signals such as environmental noise or near-end speaker's voice), the coefficients w(k,t) are miss-corrected, and the acoustic echo cannot be suppressed enough.
Accordingly, for example, as to the echo cancellation apparatus disclosed in JP No. 3877271, it is assumed that a sound volume of the second acoustic signal m(t) becomes larger when the other signals except for the acoustic echo are included in the second acoustic signal m(t). Under this assumption, the step size μ is set to be smaller for preventing the miss-correction.
However, in this echo cancellation apparatus, even if the other signals are not included in the second acoustic signal m(t), the step size μ is also set to be smaller needlessly when a sound volume of the acoustic echo becomes large. As a result, the acoustic echo sometimes cannot be suppressed quickly.